We have purified calf thymus RNA polymerase II using the polyamine P precipitation technique as the initial step. Further steps include DEAE cellulose, P cellulose and agarose chromatography. Using this procedure 5-6 mg of homogeneous enzyme IIb is prepared from 1 kg of tissue. The methods used should allow scale up for the purification of large amounts. (Hodo and Blatti, Biochemistry, 1977). Thus, we should now be able to study the size, charge density and role in catalysis of individual subunits. RNA polymerase I and III activity in uterus nuclei increase in response to estradiol administration to rats within 1.5 hrs. However, the extracted enzymes are unchanged compared to the control. Thus the control of gene expression of rRNA and tRNA in response to estradiol most likely is at the level of chromatin, not polymerase (Weil et al., J. Biol. Chem., 1977). Stimulatory factors which are specific for RNA polymerase II have been isolated and partially characterized. One factor (mol. wt. 25,000) appears to act at the elongation step. Preliminary evidence indicates that a 15,000 mol. weight subunit in the factor preparation may be a subunit of the polymerase.